In a television receiver, a comb filter is typically used for separating the incoming composite video signal into its luminance (Y) and chrominance (C) components. In a line comb filter, a pair of signals which are time delayed with respect to each other by one horizontal scanning line interval are respectively added to each other and subtracted from each other to produce the luminance and chrominance components of the composite video signal. In a frame comb filter, the signals are time delayed by one frame period (instead of one line period) and then combined to produce component signals.
Comb filters operate on the premise that the composite video signal from horizontal-line-to-horizontal-line or frame-to-frame is highly correlated. When this assumption fails, as it frequently does with program video, certain anomalies occur in the reproduced images. The anomalies result from imperfect cancellation of the chrominance signal in the luminance signal and vice versa.
For example, when a line comb filter (also known as interline or line-to-line comb filter) is used for Y/C separation, an abrupt change in the amplitude of the chrominance signal between successive horizontal lines causes scintillating serrations or hanging dots along the horizontal edges in the displayed images. The hanging dots are due to incomplete cancellation of the chrominance signal from the separated luminance signal. Likewise, incomplete cancellation of the luminance signal from the separated chrominance signal gives rise to so-called cross-color effects.
In the case of a frame comb filter (also known as interframe or frame-to-frame comb filter), motion between successive frames causes phantom or secondary images to develop around the edges of moving objects. The phantom images result when motion is present, because picture information from consecutive frames is averaged for generating the separated luminance and chrominance signals.
Motion adaptive comb filter systems have been previously proposed that combine the advantages of both the line and frame comb filters. Essentially these motion adaptive Y/C separation systems output a frame combed signal when motion is absent, and a line combed signal when motion is present.